Feature request: Ability to download sources for offline builds

A feature that would be very useful for offline or air-gapped systems would be the ability to pre-download all of the source packages. That directory containing the downloaded tarballs could then be used to perform build(s) without having to connect to the internet again. For example, something like this to download to and then consume sources from a `tarballs` dir:

```bash
./download-offline.py --sources tarballs
./build-linux.py --sources tarballs --other --options --here
```